It is generally known that antenna performance is dependent upon the size, shape, separation distance and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These parameters and relationships determine several antenna operational characteristics, including input impedance, gain, directivity, signal polarization, operating frequency, bandwidth and radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a half wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the transmitted energy. Half-wavelength antennas and quarter-wavelength antennas over a ground plane (which effectively perform as half-wavelength antennas) are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth operation, multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). Smaller packaging for state-of-the-art communications devices, such as cellular telephone handsets and other portable devices, does not provide sufficient space for the conventional quarter and half-wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing other desired antenna-operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after. Ideally, such antennas are disposed within the handset case so as to avoid possible damage to or breakage of an externally mounted antenna.
Half-wavelength and quarter-wavelength dipole antennas are popular externally mounted handset antennas. Both antennas exhibit an omnidirectional radiation pattern (i.e., the familiar omnidirectional donut shape) with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain portable communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical antenna gain is about 2.15 dBi. Antennas of this length may not be suitable for most handset applications.
The quarter-wavelength monopole antenna disposed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but when placed above a ground plane the antenna performs as a half-wavelength dipole. Thus, the radiation pattern for a quarter-wavelength monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
Several different antenna types can be embedded within a communications handset device. Generally, it is desired that these antennas exhibit a low profile so as to fit within the available space envelope of the handset package. Antennas protruding from the handset case are prone to damage by breaking or bending.
A loop antenna is one example of an antenna that can be embedded in a handset. The common free space (i.e., not above ground plane) loop antenna (with a diameter approximately one-third of the signal wavelength) displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics.
Antenna structures comprising planar radiating and/or feed elements can also be employed as embedded antennas. One such antenna is a hula-hoop antenna, also known as a transmission line antenna (i.e., a conductive element over a ground plane). The loop is essentially inductive and therefore the antenna includes a capacitor connected between a ground plane and one end of the hula-hoop conductor to create a resonant structure. The other end serves as the antenna feed terminal.
Printed or microstrip antennas are constructed using patterning and etching techniques employed in the fabrication of printed circuit boards. These antennas are popular because of their low profile, the ease with which they can be formed and their relatively low fabrication cost. Typically, a patterned metallization layer on a dielectric substrate operates as the radiating element.
A patch antenna, one example of a printed antenna, comprises a dielectric substrate overlying a ground plane, with the radiating element overlying the top substrate surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi.
Another type of printed or microstrip antenna comprises a spiral and sinuous antennas having a conductive element in a desired shape formed on one face of a dielectric substrate. A ground plane is disposed on the opposing face.
Another example of an antenna suitable for embedding in a handset device is a dual loop or dual spiral antenna described and claimed in the commonly owned U.S. Pat. No. 6,856,286 entitled Dual Band Spiral-shaped Antenna. The antenna offers multiple frequency band and/or wide bandwidth operation, exhibits a relatively high radiation efficiency and gain, along with a low profile and relatively low fabrication cost.
As shown in FIG. 1, a spiral antenna 8 comprises a radiating element 10 over a ground plane 12. The ground plane 12 comprises an upper and a lower conductive material surface separated by a dielectric substrate, or in another embodiment comprises a single sheet of conductive material disposed on a dielectric substrate. The radiating element 10 is disposed substantially parallel to and a spaced apart from the ground plane 12, with a dielectric gap 13 (comprising, for example, air other known dielectric materials) therebetween. In one embodiment the distance between the ground plane 12 and the radiating element 10 is about 5 mm An antenna constructed according to FIG. 1 is suitably sized for insertion in a typical handset communications device.
A feed pin 14 and a ground pin 15 are also illustrated in FIG. 1. One end of the feed pin 14 is electrically connected to the radiating element 10. An opposing end is electrically connected to a feed trace 18 extending to an edge 20 of the ground plane 12. A connector (not shown in FIG. 1), is connected to the feed trace 18 for providing a signal to the antenna 8 in the transmitting mode and responsive to a signal from the antenna 8 in the receiving mode. As is known, the feed trace 18 is insulated from the conductive surface of the ground plane 12, although this feature is not specifically shown in FIG. 1. The feed trace 18 is formed from the conductive material of the ground plane 12 by removing a region of the conductive material surrounding the feed trace 18, thus insulating the feed trace 18 from the ground plane 12.
The ground pin 15 is connected between the radiating element 10 and the ground plane 12. In different embodiments the feed pin 14 and the ground pin 15 are formed from hollow or solid conductive rods, such as hollow or solid copper rods.
As illustrated in the detailed view of FIG. 2, the radiating element 10 comprises two coupled and continuous loop conductors (also referred to as spirals or spiral segments) 24 and 26 disposed on a dielectric substrate 28. The outer loop 24 is the primary radiating region and exercises primary influence over the antenna resonant frequency. The inner loop 26 primarily affects the antenna gain and bandwidth. However, it is known that there is significant electrical interaction between the outer loop 24 and the inner loop 26. Thus it may be a technical oversimplification to indicate that one or the other is primarily responsible for determining an antenna parameter as the interrelationship can be complex. Also, although the radiator 10 is described as comprising an outer loop 24 and an inner loop 26, there is not an absolute line of demarcation between these two elements.